Critical elements-free magnetic materials: FeNi films synthesis and

characterization

G. Barucca(1), G. Giannopoulos(2,3), A. Kaidatzis(2), V. Psycharis(2), M. Koutsouflakis(2),

D. Niarchos(2), M. Scuderi(4), G. Nicotra(4), C. Spinella(4), S. Laureti(5), G. Varvaro(5)

(1)Università Politecnica delle Marche, Dipartimento SIMAU, Via Brecce Bianche 12, Ancona

60131, Italy; +39 0712204754; g.barucca@univpm.it (2)Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, Greece(3)Department of Engineering, University of Cambridge , CB2 1PZ Cambridge UK

(4) IMM-CNR, VII strada 5, 95121 Catania, Italy(5)Istituto di Struttura della Materia, CNR, Monterotondo Scalo, Roma, Italy

Introduction – Ferromagnetic FeNi alloy with a face-centered tetragonal (fct) L10-type structure

(tetrataenite) is a promising candidate for the replacement of high-anisotropy magnetic materials

containing rare-earths and critical elements due to its excellent intrinsic magnetic properties. The

fabrication of the L10-FeNi phase is extremely challenging and this phase is naturally found in meteorites,

where it forms over millions of years in extreme temperature/pressure conditions [1]. Different strategies

have been proposed to artificially obtain the tetrataenite phase, including deposition of alternate Fe and Ni

monoatomic layers, irradiation with neutrons or high energy electrons, addition of a third element or by

exploiting the epitaxial strain induced by suitable templates in both thin films and nanoparticles systems.

Following this last approach, the tetragonal distortion in the FeNi alloy can be induced via coherent

growth of the film on Au-Cu-Ni buffer-layers. In this work, to speed-up the search of the best Au-Cu-Ni

alloy composition, a Au-Cu-Ni compositional spread library was deposited as a buffer-layer by using a

Combinatorial Sputtering Technique to tune the lattice constant and tailor the lattice mismatch with a

FeNi film deposited on top in order to determine the best stoichiometry promoting the L10-ordering.

Interlayer atomic diffusion processes were also investigated [2].

Experimental – A high-vacuum AJA Int. ATC 2200-V magnetron sputtering system with a base pressure

of 5x10-9Torr was used to deposit on a 4-inch Si (100) substrate a Cr(10 nm)/ Cu3Au (70 nm)/combi-Au-

Cu-Ni (50 nm) / FeNi (40 nm) stack (the reported thicknesses are nominal), where the combi-Au-Cu-Ni

buffer-layer is a compositional spread layer of various stoichiometries co-deposited using combinatorial

sputtering. The final deposition of FeNi was performed by co-sputtering Fe and Ni to a stoichiometry of

50-50 at.%. The Cr and Cu3Au seed-layers were deposited at 300oC, while the combinatorial Au-Cu-Ni

and Fe50Ni50 layers were grown at 200oC; the Ar pressure was set at 3mTorr for all the layers.

Results and Discussion – Electron microscopy analysis has allowed the structure of the samples to be

investigated at an atomic level, showing that both the ordered (L10) and the disordered face-centered

cubic (fcc) FeNi phase have been sinthesized in different amounts depending on Au-Cu-Ni buffer layer

composition. Measurements have revealed that the combinatorial Au-Cu-Ni layer has the same face-

centered cubic (fcc) structure independently on his composition. This layer grows in a locally epitaxial

way on the Cu3Au seed-layer. Energy dispersive X-ray spectroscopy measurements have clearly shown

the presence of diffusion processes from the combinatorial to the FeNi layer and it is expected that the

epitaxial grow of the Au-Cu-Ni layer on the Cu3Au seed-layer is the cause of a strain-driven diffusion of

elements during the deposition. The nature and extent of the diffusion processes depend on the buffer-

layer composition and determine the amount and likely the chemical order degree of the FeNi L10 phase

formed in the different samples, thus leading to different magnetic properties.

Conclusions – This study allows providing guidelines for the preparation of the high anisotropy L10-FeNi

phase; indeed, the results suggest that including a thin diffusion barrier layer between the Au-Cu-Ni and

the FeNi layers, by keeping the crystallographic texture while reducing or even avoiding the diffusion

effect, would allow to fully exploit the potential of the Au-Cu-Ni alloy to obtain highly ordered FeNi L10

thin films.

References

[1] Poirier, E., et al. J. Appl. Phys 117, (2015) 17E318.

[2] G. Giannopoulos, et al. SCIENTIFIC REPORTS 8:15919 (2018)

M&Ns-19, Paris, 17-19 Pag. 18M&Ns-19, Paris, 17-19 July 2019